In the applications that most personal computers are used today, the computer is controlled by a keyboard through which an operator provides instructions that cause the computer to operate on selected software. In turn, and responsive to these operations, the computer provides information via video signals that are received and displayed on a cathode ray tube of a monitor. These video signals, from any particular IBM compatible personal computer, are of one type of several types of video signals, the types of video signals commonly in use today including MDA, CGA, EGA, VGA, super VGA, and XGA. For the MDA, CGA, and EGA types, the video signals consist of at least two digital, TTL level video signals for carrying the video information. A digital, TTL level horizontal sync (HS) signal and a digital, TTL level vertical sync (VS) signal are transmitted with the video signals to provide synchronization.
The VGA and XGA signals each consist of analog color red (R), green (G), and blue (B) video signals which vary from 0 volts to about 700 millivolts, with a corresponding ground return for each video signal. As with the digital signals described above, a digital, TTL level horizontal sync (HS) signal and a digital, TTL level vertical sync (VS) signal are transmitted with the color video signals, which are each provided with a discrete ground return. Additionally, three identification signals in the form of identification bits, or ID BITS, are transmitted. These ID BITS are positive 5 volt logic potentials provided by the computer on the three ID BIT lines. The monitors of each of the types of video signals are provided with means for operating on each of the ID BIT lines to selectively pull each of the +5 volt logic potentials LOW or leave it at a HIGH state, with each monitor type having a unique combination of LOWs and HIGHs of the ID BITS. The computer senses the particular coding of the "ID bits" of each type, and provides the appropriate signals for that type VGA monitor.
Typically, the video, sync signals and ground return potentials are coupled from the computer to the monitor via a multi-conductor cable, with conductors carrying the video signals, and in some instances the conductors carrying the sync signals, being micro coaxial conductors each having a discrete shield enclosing the conductor to shield the video and sync conductors from radiated interference. These cables having micro coaxial conductors therein are relatively expensive, and the coaxial conductors are more difficult to connect to terminals of plugs during the manufacturing process because the shield of each of the coaxial conductors must be separately coupled to a ground potential, typically through a pin terminal of the plug. Additionally, electrical insulation between the shield and video or sync conductor must be stripped away prior to connecting the conductor to a pin terminal or socket terminal of the interfacing plug. Further, the thin coaxial center conductors are much less durable and more prone to breakage, typically at a terminal of a connector, than ordinary stranded wire that makes up the other non-coaxial conductors of the cable.
In order to eliminate use of these more expensive video cables and the attendant problems of micro coaxial conductors, applicant has developed cabling systems as disclosed in U.S. patent application Ser. No. 07/488,710, filed on Mar. 5, 1990, and which is incorporated herein by reference, and U.S. Pat. No. 4,885,718. In these disclosures, the concept has emerged to use a cable having all non-shielded, electrically insulated, stranded conductors, and to isolate the video and sync conductors from conductors carrying the keyboard, mouse, and other digital signals, allowing a less-expensive cable to be used to collectively carry the video, mouse, and keyboard signals. This is done by arranging coupling of discrete conductors with respect to the various signals such that there are conductors in the cable carrying reference ground and power potentials interposed between the video and sync signals, and the keyboard, mouse, and other computer signals, effectively isolating the video and sync signals from the keyboard, mouse, and other signals.
With increasing popularity and use of VGA computer systems, a newer, different system for coupling analog VGA computer video signals from a source of VGA signals to equipment for directing these signals was devised, and introduced in a system for general sale to the public by Cybex Corporation of Huntsville, Ala., about four years ago. In this newer system, the cable carrying the VGA video signals is a BELDEN (TM) P/N 9935 or equivalent, which has an outer plastic sheath, a braided metallic shield just inside the outer sheath, a metallic foil shield inside the braided shield, and ten electrically insulated, color coded conductors inside the braided shield. Eight of these conductors are concentrically positioned along the inner side of the shield, and are coded, in a clockwise direction, red, green, blue, purple, orange, brown, gray, and yellow. The remaining two conductors are coded white and black, and are axially positioned within the eight concentric conductors together with cords or threads of filler material. As is commonly done in cable construction, the relative positions of the outer eight conductors spirals in one direction along the cable, with the inner two conductors twisted around each other to spiral in an opposite direction coaxially with respect to the outer eight conductors. This arrangement prevents the cable from having a tendency to curl.
For ease of identification during manufacture of video cables from this type of cable, the conductors coded red, green, and blue were selected to carry the analog red, green, and blue video signals, which as stated, are positioned next to each other in the concentrically arranged conductors. Other conductors carry the video-related signals and return potentials, with the center conductors, which are color coded black and white, carrying a video ground and the horizontal sync signal, respectively.
While this system worked relatively well, one perplexing problem was that a faint, thin, vertical white line predominantly visible on screen images having dark backgrounds appeared in the video image of some monitors, particularly those used in the IBM PS/2 Model 95 (TM) series computer systems. In spite of this, cables using this method of VGA signal transmission were successfully utilized for the next two years, until it became practical to procure custom-molded cables, at which time the use of Belden type 9935 cable was discontinued. However, the problem with respect to the faint, thin white line persisted.
The cause of this vertical white line was found after a shipment of custom-molded cable was received wherein the white conductor, which carries the HS signal, was incorrectly laid directly adjacent to the blue conductor, which carries the blue video signal. The resulting video cables manufactured from this cable caused a relatively bright blue vertical line to appear in the image of the monitor. After the incorrect conductor lay of this cable was discovered, the blue line was recognized as digital interference radiated from the horizontal sync signal conductor into the blue video signal conductor. Here, as the VGA HS signal is a digital, TTL level signal ranging from a LOW logic state of below about 0.8 volts to a HIGH logic state of above 2.0 volts, with very fast digital transitions in the low nanosecond range, it was found that the digital transitions of the HS signal were inducing interference of up to about 200 millivolts into the blue video conductor. As the blue video signal is an analog signal of only about 0-700 millivolts peak-to-peak, the induced 200 millivolt noise was more than sufficient to generate a blue line as seen on the monitor image.
With this discovery, it was then realized that the faint vertical white line seen in some systems having the correctly coded cables as described above was also caused by the horizontal sync signal, which was applied to the white, centrally located conductor. Here, the opposed spiral of the two center conductors with respect to the outer, concentric conductors brings the white conductor into close proximity with the red, green, and blue video conductors at three or four points along an eight foot video cable. This caused enough radiated interference from the horizontal sync conductor to be picked up by the R, G, and B video conductors and cause the faint white line.
It is, therefore, an object of this invention to provide a video cable for coupling computer VGA video and video related signals, and vertical and horizontal sync signals, by non-shielded, stranded conductors of a video cable such that the video conductors are shielded against radiated interference from the sync conductors by interposed conductors carrying the video related signals and return potentials.